The present invention relates generally to preventing inrush current upon the connection of a load to a power supply. The present invention more specifically relates to the prevention of inrush current upon the connection of a power supply module to a D.C. voltage, such inrush current as causes arcing, stresses components, and perturbs the input voltage distribution bus when unlimited in inrush. Also specifically, the present invention of a direct current inrush limiting circuit will prevent over-current flow from a distribution bus to a shorted load (such as a shorted power supply module), such over-current as may result in catastrophic failure of load circuitry and/or potential disruption of the distribution bus.
Diverse power-supply protection circuits exist for the prevention of damage to loads and power supply regardless of where failures originate. Practical designs usually group several of these circuits together as a dependable barrier against shorts, over-voltage, reverse voltage, and overload such as could cause component breakdown or compromise system integrity without protective circuitry. A two article series referencing fifteen different protective circuits for power supplies and their loads written by Anthony Annunziato appears as "Prevent Damage to Loads and Supplies" and "Here Are More Protective Circuits" in Electronic Design 9, Apr. 29, 1971 at page 42 and Electronic Design 10, May 13, 1971 at page 64. Specific references to active inrush-current limitors are contained in articles "Soft Starting a D.C. Power Supply Improves Reliability, Efficiency" by Roger Adair appearing in EDN, May 20, 1980 at page 7-1 and in article "Designing A Low Cost Intelligent Inrush Limiter For Off-Line Converters" appearing in proceedings of Powercon7 at page F3-1. The prior art inrush current limiting circuits contained in these and other references may be categorized as follows.
As a first inrush current limiting circuit and method to be employed in a power supply connecting a distribution bus to a load, a fixed resistor may be emplaced in series with either the input D.C. or A.C. distribution bus line. Such a fixed series resistance protects against both excessive inrush current resultant from the incipient connection of the power supply and load to the distribution bus, and again excessive current due to an internal short in either the power supply or the load or both. Such a simple solution is highly inefficient, dissipating considerable power in operation, and is of limited effectiveness in controlling current transients.
As a second inrush current limiting circuit and method, still, however, utilizing only passive resistors, a plurality of such resistors may be controllably switched in and out of series with a D.C. or A.C. distribution bus input line. Such variable series resistance protects against inrush over-current and internal shorts in either the power supply or the load. Although more efficient than a fixed resistor, such switched resistance inrush limiting circuit exhibits only limited effectiveness.
A third inrush current limiting circuit and method is applicable to connect only a D.C. distribution bus to any circuit. This method employs a series connected linear limiter circuit utilizing junction transistors or field effect transistors for protection against inrush current and internal slots. Utilization of active devices in series for control of current inrush from a D.C. bus is fast and simple. However, such active series devices exhibit high power dissipation when current limiting, and are thusly suitable only for low current applications.
A fourth inrush current limiting circuit and method utilizes silicon controlled rectifiers, or TRIACS, to connect an A.C. distribution bus input line to any circuit. Such inrush limiting circuit, as taught by Adair, McLeod, and by Annunziato as example 10 and 11, are very effective in control of inrush alternating current. Such SCR or TRIACS have no direct applicability to control of direct current inrush.
A fifth inrush current limiting circuit and method, suggested by Adair in the aforereferenced article, is to employ a thermistor in series with a D.C. or A.C. input line. Such a thermistor would protect against any large current inrush due to increased resistance at high currents. Similarly to the switched resistance method, such thermistor would exhibit only modest power dissipation during quiescent operation. However, the thermistor can protect for only one current inrush until such time as it is physically cooled off. Additionally, the thermistor offers little protection during drop-out recovery because of its long thermal time constant.
A sixth type inrush current limiting circuit and method are applicable only for the control of current input to a series pass-type regulator power supply from a D.C. distribution bus. The series pass-type regulator power supply is non-switching, and therefore does not employ that large input filter capacitor which, while dampening feedback transients onto the distribution bus, is the basic cause of high inrush current in modern switched-type power supplies. Such an input capacitor to a switched-type power supply is intentionally chosen for high storage capacity and low equivalent series resistance (ESR), thereby behaving like a nearly perfect short circuit when the supply first turns on. If however, a power supply is of the less efficient (than switching) series pass regulator type, then a sixth type inrush current limiting circuit and method utilizing a linear voltage regulator may be employed for protection against inrush current and internal shorts. In such a case the efficiency of the inrush regulating circuit is submerged in the greater inefficiency of the power supply itself.
A seventh type inrush current limiting circuit and method attempts control of the switching regulator supplied from a D.C. distribution bus in a modern-type switched power supply. Such control circuits are taught by Annunziato as his examples 12 through 14. The switching regulator control accomplished provides partial protection against inrush current and internal shorts, but does not permit of, or encompass, the utilization of any capacitive input filter on the D.C. distribution bus. If a capacitive input filter were to be employed with such an inrush current limiting circuit based on control of a switching regulator, there would be no protection against the inrush current into such filter, such as is precisely the primary inrush current in most filtered, switched-type power supplies. Additionally, there is no protection accorded by a circuit controlling inrush current via control of the switching regulators for those inrushes occurring due to the turn-on of switching transistors within such switching regulators.